In the prior art, a method is known by which the SOC (State Of Charge) of a lead battery is estimated from the OCV (Open Circuit Voltage) thereof in parking of a vehicle (the ignition being turned off) by using a fact that the SOC and the OCV have approximately a linear relation.
However, due to polarization occurring in the lead battery during the vehicle travelling, the OCV is not stabilized immediately after parking of the vehicle. As a result, actual measurement of the SOC is carried out after the elimination of the polarization voltage after the parking. Thus, a long time may be required until the SOC can be measured. For example, a method has been proposed where a lead battery is discharged for a part of the remaining capacity, then a predetermined time is measured for the elimination of polarization, and thereafter, the OCV of the lead battery is measured, or so. However, the measurement accuracy may be degraded (for example, see Patent Reference No. 1).